If your life total drops to 0 or less, it’s too late to use this ability before losing the game.

9/25/2006

The life you gain is based on the total of all changes where your life total went down during the turn, not the net downward change. So if you lose 5 life, gain 3 life, and then lose 2 more life before activating this ability, the ability causes you gain 7 life, not 4.